1. Field of the Invention
The present invention relates to a key sheet for a pushbutton switch for use in an operating portion of various apparatuses such as a mobile phone, a PDA, a car navigation apparatus, and a car audio apparatus. In particular, the present invention relates to a key sheet suitable for an application in which a plurality of key tops are exposed for use through an operation opening which is formed in the casing of an apparatus without a partition frame.
2. Description of the Related Art
Like a mobile phone 1 shown in FIG. 13, due to requirements in terms of miniaturization of the entire device or an operating portion thereof, artistic design properties, and the like, it has been desired to provide a pushbutton switch in which plural key tops 3 of a key sheet 2 (FIG. 14) arranged at a narrow spacing are exposed through an operation opening 1b which is formed in a casing 1a without a partition frame. As shown in FIG. 14, the key sheet 2 of the background art is constructed of a plurality of (seventeen in total) key tops 3 firmly attached to abase sheet 4 made of silicone rubber. Specifically, the key sheet 2 includes a large key top 3a situated in the upper middle section thereof and used for directional input in the vertical and horizontal directions, four small key tops 3b situated on the right-hand and left-hand sides of the key top 3a, and twelve medium-sized key tops 3c situated below the key tops 3b. The gaps between the adjacent key tops 3a, 3b, and 3c are extremely narrow, the key tops being arranged at a spacing, for example, as small as approximately 0.15 mm to 0.2 mm. The gap between the key tops and the operation opening 1b is also very narrow, its dimension being approximately the same as mentioned above. An art relating to such a key sheet 2 with a narrow key top spacing is disclosed in, for example, Japanese Patent Application No. 2003-114833.
As shown in FIG. 15, in mounting the key sheet 2, the outer edge portion of the base sheet 4 is held in press contact with structural elements inside the casing 1a over the entire circumference thereof. In the case of this background art, those structural elements are the opening edge portion of the operation opening 1b on a back surface 1c of the casing 1a, and a circuit board 1d built in the casing 1a. According to this mounting structure, on the inner side of the press-contact portion, the key sheet 2 is not locked with respect to the casing 1a and the circuit board 1d. Accordingly, when the key sheet 2 is set upright or is laid face down during use of the mobile phone 1, the base sheet 4, which includes a rubber-like elastic body such as flexible silicone rubber, may expand in its entirety under the weight of the key tops 3 to be there by distorted. This may lead to positional deviation between pushers 4a of the base sheet 4 and contact switches 1e that are provided on the circuit board 1d and composed of metal disc springs and a contact circuit, resulting in operation failures such as the inability or difficulty of effecting input even when the key tops 3 are depressed. Further, depending on the manner of distortion of the base sheet 4, the depression stroke for input may differ among the key tops 3, which may adversely affect the operability. This may also impair the appearance of the mobile phone 1. Moreover, one of adjacent key tops 3 may slip into the space under the other.
The above-mentioned problems, which arise due to the distortion of the base sheet 4 fabricated of a flexible rubber-like elastic body, can occur whenever two or more key tops are arranged per one operation opening. Further, the slippage of one key top under the other can occur not only in the case of a portable apparatus such as the mobile phone 1 but also in the case of a stationary apparatus. Therefore, a solution to these problems has been demanded.